The present invention relates to a system for transmitting power of an engine to four wheels of a four-wheel drive motor vehicle having a central differential, and more particularly to a system which may increase the number of the speed ranges of an automatic transmission.
A conventional four-wheel drive motor vehicle has the central differential having a functional control device for restricting the differential operation and for controlling distribution of an output torque of the transmission to front and rear wheels of the vehicle. Recently, the central differential is used for variably controlling the torque distribution, or used as a reduction gear or speed-up gear for the automatic transmission, thereby improving driveability, operability, stability and braking effect of the vehicle.
Japanese Patent Application Laid-Open 4-27622 discloses the four-wheel drive motor vehicle providing the central differential which comprises a complex planetary gear device and a control device is provided. The control device comprises first and second fluid operated multiple-disk friction clutches and a brake for a 5th speed for providing a 5th speed. The first clutch is provided for transmitting the power of an engine from a transmission to front and rear wheels. The second clutch is provided for restricting the differential operation of the central differential and variably distributing the output torque of the transmission to the front and rear wheels.
However, in such a system, the torque distribution function and differential operation function are limited by the 5th speed providing system.
Furthermore, it is desirable for be capable of using such a central differential for a two-wheel drive motor vehicle such as a front-wheel drive motor vehicle having a front-mounted engine.